The underground air from caves has been known for centuries to have a very uniform temperature and to be useful for maintaining such items as wines and cheeses at desirably even temperatures. Many attempts have been made to use this uniform underground temperature to advantage in heating or cooling aboveground structures, and such efforts are exemplified in U.S. Pat. Nos. 1,830,099, 1,974,244, 2,077,406, 2,119,038, 2,217,190, 2,554,661, 2,564,215, 2,628,374, 2,680,354 and 2,793,509.
So far as is known, there is no generally available and commercially successful heating, ventilating, and air conditioning or air treatment system using underground air. The known prior art has not disclosed the features of the present invention, but has relied on outdoor air passing through an underground duct as the source and sink for heat for heating and cooling a space, whether direct or by means of an intermediate heat pump. The prior art has ignored the fact that outdoor air or air recirculated through an underground duct is often more advantageous for heat exchange purposes than outdoor air after passing through the underground duct, and that less use of the underground duct also allows it to be reconditioned, rejuvenated, or recharged as a heat source or sink by passing outdoor air therethrough and back to the outdoors when the underground duct is not being used as the heat source or sink.
On the other hand, the present invention provides methods and apparatus for automatically heating or cooling a space by use of air as the heat exchange medium, whether outdoor air direct or through an underground duct, or air recirculated through an underground duct, whichever is the most advantageous source or sink for heat, and for reconditioning the underground duct for further and more advantageous use during periods when it is not being used as a heat source or sink but outdoor air is being used directly therefor. The present invention also provides for removal of latent heat from underground duct air, recirculated air, or outdoor air when they are used directly and exclusively for cooling a conditioned space without use of mechanical refrigeration apparatus for removal of sensible heat from the air. Such use of the most advantageous heat sources and sinks should result in substantial economies in use of electrical or other power for heating, ventilating, and air conditioning purposes, and should also require less underground duct work than would be required by the apparatus of the prior art.
Furthermore, such use of the most advantageous heat sources and sinks should provide a more even load on the compressor of a heat pump in the system, thereby prolonging its useful life. Use of water, including collected rainwater, to permeate the ground around the underground duct, might provide improved heat transfer between duct and ground and within the ground, as mentioned in the aforesaid patents.